The present invention pertains to venting devices incorporated into containers that are used in a pressurized condition or become pressurized through use or malfunction. Particularly, the present invention pertains to a safety vent that allows increased venting during incidental overpressure conditions that may occur during use of rechargeable electrochemical cells.
Due to improved performance and energy density, rechargeable electrochemical cells have been increasingly used as a power source for a great variety of devices. These secondary cells can be classified as either "vented" or "sealed" in design and operation. Vented cells release gases during the normal operating processes of use. A low pressure vent mechanism is incorporated into many vented cells to allow such a release. Sealed cells are typically pressurized and do not vent under normal operating conditions. Sealed cell operational pressures are a result of gas generation inherent in the chemical processes used for storing and discharging electrical energy. For example, in a secondary cell using a nickel-metal hydride based chemistry, the operational pressures due to hydrogen gas production may be in the range of 90 to 100 psi (6.3 to 7.0 kg/cm.sup.2). However, most sealed cells are still designed with a venting mechanism. This is a safety feature intended to accommodate the very high pressures which may result from malfunction or unintended operating conditions. In sealed cells, such as nickel-cadmium or nickel-metal hydride cells, conditions such as unintended overcharging can produce extremely high pressures that are potentially a safety hazard to the user. These pressures may exceed 1000 psi (70.3 kg/cm.sup.2) in nickel-metal hydride cells. Initially, pressure in these cells is due to gas generation inherent in the chemistry of the cell. If the gas pressure reaches the designed venting pressure and is successfully released, the pressure will be limited. If gas generation exceeds the venting capacity or if vent clogging occurs, internal pressures can quickly increase. This is exacerbated by high heat generation during charging and more particularly by shorts occurring between the cell electrodes causing high currents. Because of the pressures involved, catastrophic failure with rapid release of energy (explosion) may occur if venting is not successful. Various vent designs have been proposed in the prior art to release the pressures in these cells. Typically, they include a sealing element such as a metallic spring or elastomer that is preloaded. The level of preload is set such as to be overcome only by a pressure exceeding a predetermined safety limit. These are generally resealable seals. In the past, non-resealable "one-time" seals such as burstable diaphragms have also been used. One design of these vents incorporates a diaphragm and a cutting element which are forced together by high internal pressures. When these pressures exceed a limit, the cutting element breaks the diaphragm, allowing venting of gases through the created opening. The obvious disadvantage of such a "one-time" seal is the lack of resealability. After the diaphragm is broken, it is impossible to reseal and repressurize the container to operating pressures. For this reason and others, resealable seals in vents predominate current electrochemical designs. In a resealable seal, a sealing element is typically deformed or moved to create a venting area. The sealing element deformation is elastic, that is reversible, such that at elimination of the overpressure the seal is reformed to again maintain operating pressures. Resealable seals and their desirability are discussed in U.S. Pat. Nos. 5,258,242 to Dean et al. Various designs of resealable seals for electrochemical cells are also disclosed in the U.S. Pat. Nos. 5,171,647 to Dean et al.; 4,298,662 to Sugalski et al; and 4,271,241 to Hooke et al. Many other variations of resealable seals using metallic springs or elastomer sealing elements are provided in published patents.
A basic presumption with these and other previous electrochemical cell vents is that the medium being vented is gaseous in nature. This is due to a focus on the electrochemical reactions and inherent gas formations in the storage and release of electrical energy in these cells. With the current designs and operating procedures being used, particularly in metal hydride cells, this is no longer a proper approach. Because of the capacities and charge and discharge rates possible with newer cell designs, such as nickel-metal hydride cells, new physical conditions beyond operational conditions must be considered. Many nickel-metal hydride cells are charged at rates of several amperes. Discharge rates also may be at these high levels. Accidental overcharging at these rates can result in physical changes to cell materials not experienced by previous cell designs. High temperatures and pressures and large generated gas volumes can cause cell contents to be forced into vent openings. Plastic non-conductive separators, insulating tapes and even active material from the electrodes may be forced out of the cell container. In these circumstances, vents designed to release gaseous matter may be quickly clogged and no longer able to function to reduce pressure. Clogging or inability to vent adequately is often due to insufficient vent area or disabling of the vent opening mechanism. The vent area provided by elastomeric sealing elements is relatively very small--very little area is required to vent low volumes of gas. The elastomer sealing element is also often captured by an effectively rigid structure that does not allow an increase of vent area if needed. Where metallic springs are employed, alone or in conjunction with elastomeric elements, other problems can exist. Metallic springs have the potential to provide large increases in vent area. However, with these designs, the spring elements can cause contact freezing and solidification of the hot liquefied contents of the cell at initial release. Clogging or disabling of the spring element may result. In a cell that cannot vent solids during extreme conditions, pressures overwhelming the integrity of the cell container may be created. For these reasons, prior resealable vent designs are inadequate for many of today's secondary cells.
To ensure integrity of new electrochemical secondary cells, various standard tests have been developed in the industry. One is a "hot plate" test where cells are heated by immersion in a high temperature medium. The resulting thermal expansion of the solid and gaseous contents of the cell creates pressures exceeding normal operating pressures. The high temperatures can cause any plastic components with a low melting point to become fluid and venting of these types of solids is not unusual in this test. A hot plate test temperature of 280 degrees centigrade is typical. Also, to simulate conditions of an accidental overcharge, a "continuous overcharge" test is being used in the industry. This test is particularly applicable to cells normally charged at high rates for short durations where overcharging can result in rapid gas and heat production. Individually, or in a battery, cells are subjected to a continuous high ampere charging circuit. In these situations, where the large amounts of energy being supplied cannot be absorbed, energy dissipation in some form is required. Venting is essential to dissipate energy and limit internal pressures. Expulsion of solid matter through venting is normal during this test. In both the hot plate and continuous overcharge tests, cell success is typically judged by the integrity of the cell container. Most prior vent designs are inadequate for these test conditions.
An additional objective of secondary cell design is to maximize the useful volume of the cell container and thereby maximize capacity. The overall dimensions of commercial electrochemical cells are relatively fixed by specific standards. To maximize the capacity or energy density of a specific cell, the portions of the cell volume used by non-active elements such as vents and seals must be minimized. For this reason, safety vent designs attempt to minimize the dimensions of the vent and seal elements. However, this makes it more difficult to provide a vent device with the larger vent areas needed to allow escape of solids.
What is needed is a safety vent that can operate at the high pressures of many present day electrochemical secondary cells and can vent gaseous and solid matter during extreme conditions while maintaining the integrity of the cell structure. This same vent must be sufficiently compact that it does not reduce the capacity of the cells.